One of my neighbors (not sure which one) has a cat that keeps defecating in my yard, I keep finding remnants every other day.
While amusing, I don't have time to build the motion activated scare machines depicted in these videos
However, commercial, and home made scented products seem to have mixed reviews:
Further, it seems that using a low voltage electric fence to deter cats and dogs is not allowed in the US.
Well, the quickest thing that comes to mind is the possibility of using a litter attractant at your property line. Basically, these are designed to attract cats to use litter boxes rather than undesirable locations, but you could look at that as an option for encouraging the cat onto his own property.
Now, this stuff isn't specifically intended for outdoor usage, so rain and other weather probably means a more frequent application, but if you know that you're in for a dry spell, then it might be worth a shot.
You could also put out a sheltered litter box and use the attractant in it. While it would still mean cleaning it up, at least it would be in a controlled location and less likely to be discovered through more unpleasant means...
There are a bunch of potential solutions, but in the end, unless you can keep an animal from entering your yard, you can't keep them from defecating/urinating in it. Even this solution is not sufficient for birds who fly over, nor for animals who broadcast across and through fence.